# Help w/ Y-Intersect Formula

**URL:** <https://forum.kirupa.com/t/help-w-y-intersect-formula/296925>\
**Category:** flash\
**Created:** [September 22, 2009, 7:50pm UTC](https://forum.kirupa.com/t/help-w-y-intersect-formula/296925 "2009-09-22T19:50:32Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![shade\_v2](https://avatars.discourse-cdn.com/v4/letter/s/a9adbd/32.png) [@shade\_v2](https://forum.kirupa.com/u/shade_v2)\
**Post date:** [September 22, 2009, 7:50pm UTC](https://forum.kirupa.com/t/help-w-y-intersect-formula/296925/1 "2009-09-22T19:50:32Z")

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Apologies if I’m off forum. Nothing seemed to fit.

I recently educated myself on how to find the intersection of two lines and I’ve been stumped by a certain formula found at [http://www.euclideanspace.com/maths/geometry/elements/intersection/twod/index.htm](http://www.euclideanspace.com/maths/geometry/elements/intersection/twod/index.htm).

There the formula for y-intersect, when two points of a line are known, is presented as:

```auto
b = (B.x*A.y - A.x*B.y)/(B.x - A.x)

```

I follow the author’s algebra, terse as it is, all the way from the basic line-formula right up to the formula for slope:

```auto
m = (B.y - A.y)/(B.x - A.x)

```

But I don’t see what he’s done from this point to bring about this 2-point formula for y-intercept, and he doesn’t bother trying to explain.

I can see that the numerator of slope has been inverted and multiplied by the x values of A and B, but I haven’t been able to understand, algebraically, why. So the answer I’m looking for would bridge the gap in these formulas in a plain, step by step, fashion. Thanks.